Packaging materials for packaging various articles such as foods are often required to have barrier properties against gases such as oxygen. The use of a packaging material having poor gas barrier properties may, for example, cause oxidation by oxygen or food decay by proliferation of microorganisms, thus leading to deterioration of the packaged article. Therefore, conventional packaging materials generally include a gas barrier layer for preventing transmission of oxygen etc.
Examples of such a gas barrier layer include layers made of vinyl alcohol polymers (e.g., polyvinyl alcohol and ethylene-vinyl alcohol copolymer). The layers made of these vinyl alcohol polymers have the advantages of being transparent and being less problematic in terms of disposal. However, the layers have the disadvantage of being poor in water vapor barrier properties.
A film in which a metal oxide (silicon oxide, aluminum oxide, magnesium oxide, or the like) is deposited on a polymer film is known as a gas barrier layer having barrier properties against oxygen and water vapor. However, the deposited metal oxide layer may be cracked due to deformation of or impact on the packaging material, resulting in marked deterioration of the gas barrier properties. In order to suppress the deterioration of barrier properties caused by deformation of or impact on a packaging material, JP 2006-175784 A (Patent Literature 1) discloses a gas barrier film in which a protective layer made of an organic compound is formed on an inorganic deposited layer. In the case of this film, however, the deterioration of gas barrier properties is not sufficiently suppressed.
As an example of employing a coating layer containing a phosphorus compound or a silicon compound, JP 2008-516015 T (Patent Literature 2) discloses a method for forming a coating layer using a solution in which an aluminum salt and a phosphoric acid ester are contained in an organic solvent. In addition, WO 2009/125800 A1 (Patent Literature 3) discloses a method for forming a coating layer using a solution containing silane and aluminum alkoxide.